Recently, a secondary battery, which can be charged and discharged, has been widely used as an energy source for wireless mobile devices. In addition, the secondary battery has attracted considerable attention as a power source for electric vehicles (EV) and hybrid electric vehicles (HEV), which have been developed to solve problems, such as air pollution, caused by existing gasoline and diesel vehicles using fossil fuels.
Small-sized mobile devices use one or several battery cells per device. On the other hand, middle or large-sized devices, such as vehicles, use a middle or large-sized battery module having a plurality of battery cells electrically connected to one another because high power and large capacity are necessary for the middle or large-sized devices.
Preferably, the middle or large-sized battery module is manufactured so as to have as small a size and weight as possible. For this reason, a prismatic battery or a pouch-shaped battery, which can be stacked with high integration and has a small weight to capacity ratio, is usually used as a battery cell of the middle or large-sized battery module. In particular, much interest is currently focused on the pouch-shaped battery, which uses an aluminum laminate sheet as a sheathing member, because the pouch-shaped battery is lightweight, the manufacturing costs of the pouch-shaped battery are low, and it is possible to easily modify the shape of the pouch-shaped battery.
In order for the middle or large-sized battery module to provide power and capacity required by a specific apparatus or device, it is necessary for the middle or large-sized battery module to be configured to have a structure in which a plurality of battery cells is electrically connected in series to each other or in series and parallel to each other and the battery cells are stable against external force.
Meanwhile, the battery cells constituting the middle or large-sized battery module may be secondary batteries which can be charged and discharged. Consequently, a large amount of heat is generated from such high-power, large-capacity secondary batteries during charge and discharge of the secondary batteries. If the heat, generated from the unit battery during charge and discharge of the unit battery, is not effectively removed from the unit battery, the heat accumulates in the unit battery with the result that deterioration of the unit battery is accelerated. According to circumstances, the unit battery may catch fire or explode. For this reason, a battery pack for vehicles, which is a high-power, large-capacity battery, needs a cooling system to cool battery cells mounted in the battery pack.
In a middle or large-sized battery pack including a plurality of battery cells, on the other hand, the deterioration in performance of some battery cells leads to the deterioration in performance of the entire battery pack. One of the main factors causing the non-uniformity in performance is the non-uniformity of cooling between the battery cells. For this reason, it is necessary to provide a structure to secure cooling uniformity during the flow of a coolant.
As shown in FIG. 1, a conventional middle or large-sized battery pack may be configured to have a structure in which unit cells are uprightly arranged in a width direction W of a battery pack 20 to constitute a battery module 10 layer and, for example, four battery module 10 layers are mounted in a pack case 15 in a state in which the battery module 10 layers are stacked in a height direction of the battery pack 20.
Consequently, a coolant, introduced from a coolant introduction part 11, which is formed at the upper part of the battery pack, sequentially passes through flow channels defined between the respective unit cells stacked in a vertical direction to cool the unit cells and is discharged from the battery pack through a coolant discharge part 12, which is formed at the lower part of the battery pack.
In the above structure, however, temperature deviation between the battery module layers is generated and the coolant flow channel is lengthened to generate differential pressure. As a result, it is difficult to achieve uniform cooling between the battery cells.
Consequently, there is a high necessity for technology to fundamentally solve the above problems.